The electrons repel each other, so, like the coils of a compressed spring, they have potential energy.
As the electrons slowly flow round the circuit, they transfer energy from the cell to the lamp.
The energy is radiated by the hot filament.
Potential Difference Across a Circuit
A cell normally has a voltage marked on it.
The higher its voltage, the more energy it gives to the electrons pushed out.
he scientific name for voltage is potential difference (p.d.).
P.d. can be measured by connecting a voltmeter across the terminals of the cell.
The SI unit of p.d. is the volt (V):
If the p.d. across a cell is 1 volt, then 1 joule of potential energy is given to each coulomb of charge.
In other words, 1 volt means 1 joule per coulomb (J/C).
If the p.d. across a cell is 2 volts, then 2 joules of potential energy are given to each coulomb of charge, ...and so on.
A cell produces its highest p.d. when not in a circuit and not supplying current.
This maximum p.d. is called the electromotive force (e.m.f.) of the cell.
When a current is being supplied, the p.d. drops because of energy wastage inside the cell. For example, a car battery labelled ‘12V’ might only deliver 9V when being used to turn a starter motor.
Cells in Series
To produce a higher p.d., several cells can be connected in series (in line) as shown below.
The word ‘battery’ really means a collection of joined cells, although it is commonly used for a single cell as well.
Potential Difference Across a Circuit
Like the battery, each lamp has a p.d. across it:
If a lamp (or other component) has a p.d. of 1 volt across it, then 1 joule of potential energy is spent by each coloumb of charge passing through it.
The second diagram shows the same circuit with voltmeters connected across different sections (the voltmeters do not affect how the circuit works). The readings illustrate a principle which applies in any circuit: